An essential optimisation objective for the use of lithium ion batteries is to increase energy density. Energy density is determined on the material level by working potential and specific capacitance. At the level of the cell, the packing density of the active material powder is important.
The layer oxides that are typically used as cathode active material in commercial lithium ion cells, lithium cobalt oxide LiCoO2 (LCO), LiNi1/3Mn1/3CO1/3O2 (NMC) and LiNi0.85CO0.10Al0.05O2 (NCA) achieve specific capacitance values between 150 and 180 mAh/g, the lithium manganese spinel LiMn2O4(LMO) is used with practical values of about 110 mAh/g.
Lithium-Nickel-Manganese High-Voltage Spinels
Manganese- and lithium-rich layer oxides of the type xLi2MnO3.(l−x)LiMO2 (in which M is typically Ni, Co and Mn) are being developed as promising materials for the next generation, but are not currently being used. They achieve considerably higher specific capacitance values of 250-280 mAh/g than the materials which have been in commercial use up to now. However, these peak values are only achieved with low current rates. Even at a moderate current rate of 1 C the capacitance values fall below 180 mAh/g and into the range of the layer oxides which are already in commercial use. They also contain the expensive element cobalt. A further marked disadvantage of this material class is its structural instability: as cycles progress, the layer structure changes increasingly into a spinel-like structure, which results in a significant reduction of the working potentials and and therewith also a loss of energy density. No approaches to solve this situation have yet been identified.
Lithium-nickel-manganese based transition metal oxide particles of the spinel type (LNMS) are thus promising electrode materials in lithium ion batteries. In particular, they have a high voltage plateau of 4.7 V vs. Li/Li+. The redox principle of electrochemical conversion can be described using the compounds having formula Li1.0NixMn2-x—O4-δ. One electron/lithium ion may be replaced per formula unit, which corresponds to a specific capacitance of 147 mAh/g. For purposes of stoichiometry, both the nickel content x and the oxygen content 5 may be varied. The distribution of the formal oxidation levels and the resulting electrochemical voltage curve of the materials is described with the formula Li[Ni(II)xMn(III)1−2x+2δMn(IV)1+x−2δ]O4−δ. Two voltage plateaus may occur depending on x and δ: a) a plateau at 4.1 V vs. Li/Li+, which can be associated with the Mn(III)/Mn(IV) redox pair, with utilisation of (1−2x+δ) electrons/lithium ions per formula unit; b) a plateau at 4.7 V vs. Li/Li+ for the Ni(II/(IV) redox process, with utilisation of (2x−2δ) electrons/lithium ions per formula unit. The fully substituted phase Li[Ni(II)0.5Mn(IV)1.5]O4.0 is obtained with x=0.5 and δ=0, with as single voltage plateau at 4.7 V vs. Li/Li+.
The working voltage of the LNMS is much higher than the materials currently in commercial use, and so contributes proportionally to the increase in the energy density of the cell. In order to be able to fully exploit the energy density contribution of the LNMS in the battery cell, it must be possible to pack the materials as densely as possible in the cell. For this in particular, a high powder density is required. It is thereform important to produce the materials in such a manner that they have a high powder density and suitable shape and size distribution. At the same time, the production process must be simple and inexpensive.
Production Process for Lithium-Nickel-Manganese High-Voltage Spinels
The related art includes descriptions of various production methods for lithium-nickel-manganese high-voltage spinels.
In sol-gel synthesis, colloidal dispersions called sols are produced from soluble reactants, and are transformed into a solid, three-dimensional network, the gel, by ageing. The gel is a chemical precursor of the product. The products have a small crystallite size, which increases current carrying capacity and is therefore desirable. At the same time, they have a large surface area, which encourages the undesirable side reactions, particularly in high-voltage applications. However, the production method is very expensive. It is therefore not reasonable to raise the production method to industrial dimensions. The method is therefore employed mainly in the purely scientific-experimental context.
Besides the above, there are also descriptions of pure solid synthesis variants, which are used in different ways in the production of battery materials. In particular, they are also important as cathode material in the technical production of LiCoO2. In the method, oxides, carbonates or other crystalline starting compounds are mixed with each other and then subjected to heat treatment. The particles used typically have high density and low porosity. The reactants need to be pulverised individually or together with each other to reduce the particle size, ensure the most homogeneous particle distribution possible in the mixture, and so shorten the diffusion paths in the calcining step. In contrast to the sol-gel method, the particles used are typically microscopically small, so the diffusion paths for the reaction are relatively long. In order to accelerate and complete the conversion, the reactions are conducted at high temperatures, possibly as high as 800° C. to 900° C. for LNMS. Calcining temperatures are a critical parameter for the LNMS: the nickel-manganese spinel tends to release oxygen from the lattice at temperatures above 700° C. This leads to the formation of oxygen-poor phases, wherein nickel oxide NiO may be precipitated from the lattice as a separate phase. This process is reversible, however. If high temperature calcining is followed by a healing process at 700° C. an, the NiO may be reassimilated in the lattice, and the oxygen gaps may be filled. In this way, the desired target stoichiometry may be set at the crystalline level.
From the technical perspective, however, this healing process is associated with a great deal of time, and consequently cost, if all impurities that interfere with the electrochemical behaviour are to be removed from the lattice. The samples obtained in this way yield greater densities than those from the sol-gel process, and have a smaller specific surface. Grinding processes are necessary afterwards to adjust the materials to the desired particle size specification, and these in turn make the technical process more expensive. Moreover, it is not possible to entirely prevent impurities from being introduced during the grinding process.
In both of the methods described, the sol-gel and solid synthesis method, extremely pure starter materials must be used. The stringent requirements regarding quality and purity of the raw materials raise the costs of producing the raw materials.
Besides these methods, combination methods are also described, in which a precursor, e.g., a transition metal carbonate, oxide or hydroxide is prepared by precipitation and then reacted with stoichiometric quantities of a lithium compound to obtain the end product. The purity requirements applicable to the reactants in this method are not as stringent, since soluble impurities are flushed out in the filtration and washing process of the precipitate and so do not remain in the product. Such spherical materials are also described for the class of LNMS. However, the maximum powder densities achieved in the products are low.
Wang et al. (Journal of Power Sources, 274 (2015) 451-457) reported with reference to a parameter screening that tamped densities in the range of 0.7-1.5 g/cm3 can be obtained by precipitations of hydroxides of the compound Ni0.25Mn0.75(OH)2 in the presence of NH3. The secondary agglomerates are constructed from flake-like primary crystallites which are arranged in the grain like a house of cards. This arrangement results in the formation of corresponding cavities in the grain, which explain the low tamped density values. High temperatures would have to be used when sintering the particles to obtain dense grains; this would again lead to crystallite growth and undesirable segregation of NiO.
Over-Lithiated Transition Metal Oxides
With regard to use in battery cells, it is typically necessary during assembly to ensure that the positive and the negative electrode have the same charge state. If it is intended to use active material in the form of Li[Ni(II)0.5Mn(IV)1.5]O4.0 and also use the lithium-rich phases at the same time, the counter electrode may be brought to the same charge state by prelithiation, for example. In this case, both electrodes would be assembled in the partially charged state.
If it is not possible to pre-lithiate the anode easily because of the associated technical conditions, it may be helpful to supply the material for cell construction in over-lithiated form Li1+x[Ni(II)0.5Mn(IV)1.5]O4.0 (0<x<1.5). In this case, it is not necessary to pre-lithiate the negative electrode, and assembly is carried out in the fully discharged state. The over-lithiated material Li1+x[Ni(II)0.5Mn(IV)1.5]O4.0 may advantageously also be introduced as an additive in cells with anode materials such as silicon, amorphous carbon or other comparable materials which are subject to high, irreversible starting losses. In such cases, the lithium excess x of the cathode material may be used to compensate for the losses at the anode. After the compensation, the additive functions as cathode material on the high-voltage plateau in der cell.
Therefore, it is important to provide materials with high capacitance and good load-bearing capability, and at the same time to produce them in such manner that they have high tamped densities and a suitable shape and size distribution. Moreover, it is important a implement a process with which the high-density materials can easily be converted to their over-lithiated form. Additionally, all steps in the production process should be inexpensive and easy to put into practice.
Particularly promising electrode materials in lithium ion batteries are over-lithiated spinels of the type Li2M2O4. The chemical preparation of these is based on the production of a spinel with starting composition of LiM2O4. This is converted in a subsequent reduction process in the presence of a lithium source. Since these compounds undergo structural changes at higher temperatures, reactants and process conditions must be selected such that the critical temperature is not exceeded.
Amine et al. (J. Electrochem. Soc., Vol. 143 (1996) No. 5, 1607-1613) produce a finely particulate LiNi0.5Mn1.5O4 by thermal decomposition of corresponding acetates with the sol-gel synthesis method. The product obtained is reacted with a solution of lithium iodide in acetonitrile. A sixfold excess of lithium iodide is needed. The product stoichiometry is indicated with Li2Ni0.5Mn1.5O4. The working potential of this material is 3 V vs. Li/Li+. A maximum capacitance of 170 mAh/g is achieved; the capacitance falls to 75% of the initial capacitance within 30 cycles. The disadvantage of this method is that very large excesses of lithium iodide are needed as well as the use of toxic solvents. The required reaction time of 13 h is long, expensive methods must be employed to dispose of the solvent and the excess lithium iodide.
West et al. (Electrochimica Acta, Vol. 45 (2000) 3141-3149) produce Li2Mn2O4 by converting LiMn2O4 in a lithium iodide melt. The reaction takes five hours, the temperature is 460° C. This temperature is slightly higher than the melting point of lithium iodide (446° C.). No information is provided regarding the production and specification of the starter material LiMn2O4. Nickel-containing materials are not described. In the first two cycles, about 1 lithium is replaced per manganese. A 4.7 V plateau does not exist. No information is provided about cycle stability. Peramunage et al. (J. Electrochem. Soc., Vol. 145 (1998) No. 4, 1131-1136) use a Li1.1Mn2O4 which is prepared by thermal conversion of MnO2 with LiOH as the starter compound. For lithiation, the pre-dried material is introduced into dry hexane. Stoichiometric quantities of 2.5 M solution of butyl-lithium in hexane are added slowly with vigorous agitation. The Li2Mn2O4 produced is then washed with hexane and dried. Electrochemically, 1.4 Li/Mn2 are used, which corresponds to a specific capacitance of 198 mAh/g relative to Li2Mn2O4. About 60% of this capacitance is discharged on a plateau at 4 V vs. Li/Li+, then 20% of the capacitance each at 2.8 V and 2.2 V vs. Li/Li+.
Park et al. (Electrochemical and Solid-State Letters, 8 (2005) A163-A167) prepare a LiNi0.5Mn1.5O4 from the corresponding metal nitrates using an ultrasonic spray pyrolysis process. The authors demonstrate that LiNi0.5Mn1.5O4 can be cycled electrochemically on the 3 V plateau (LiNi0.5Mn1.5O4↔Li2Ni0.5Mn1.5O4). A chemical or electrochemical conversion to Li2Ni0.5Mn1.5O4 is not described.
All of the lithiation methods described are extremely complex. They either require critical reaction media such as butyl lithium, toxic solvents, or high temperatures, and they are therefore not suitable for conversion on a large industrial scale.
Accordingly, there is a need for methods to produce lithium-nickel-manganese based transition metal oxide particles, particularly over-lithiated lithium-nickel-manganese based transition metal oxide particles, in which the drawbacks of the methods described in the related art have been overcome. In particular, the materials obtained should be constructed from small crystallites to allow rapid discharging/charging kinetics; however, small crystallites usually require low conversion temperatures, and the processes which are suitable for them typically do not result in the requisite grain densities. At the same time, however, the grain density of the materials should be high to enable high charging plateaus in the cells. Moreover, the specific surface area should be low to suppress side reactions, something which has only been achieved with high temperature processes in the related art.